General purpose polystyrene (GPPS) is made from styrene, a vinyl aromatic monomer that can be produced from aromatic hydrocarbons, for example those derived from petroleum. GPPS is useful in a variety of applications, such as casing for appliances, molded into toys or utensils, or expanded to create foamed styrene. In most cases, GPPS is a hard and brittle plastic, however, the use of comonomers may alter its physical properties, for example, styrene can be copolymerized with polybutadiene to make SBS polymer. The resulting SBS polymer has more rubber-like qualities, such as elastomeric performance and abrasion resistance. Other polymers can also experience altered physical properties when polymerized using comonomers. Ionic comonomers, for example, may alter the properties of a polymer, such as melt flow rate, melt strength, polydispersity, and glass transition temperature.
When ionic comonomers are used, the polymer product can be referred to as an ionomer. An ionomer is a polymer that contains nonionic repeating units and a small portion of ionic repeating units. Generally, the ionic groups make up less than 15% of the polymer. The ionic groups are attached to the polymer backbone at random intervals and can reversibly associate with one another, creating reversible crosslinks. These reversible crosslinks can cause the product polystyrene to be less brittle and more resistant to abrasions. Ionic aggregates in the copolymer can also affect such properties as bending modulus, tensile strength, impact resistance, melt strength, glass transition temperature and melt viscosity.
Unsaturated carboxylic acid salts are a group of ionic comonomers that can serve as effective crosslinking agents. Metal methacrylates are an example of carboxylic acid salts. One metal methacrylate that may be useful as an ionomeric crosslinker is zinc dimethacrylate, Zn(MA)2. It is a divalent metal and therefore, capable of forming two reversible crosslinks with the ionized acid ends of the methacrylates that are incorporated into the backbones of polystyrene chains. Zinc has the disadvantage of being listed on the EPA list of hazardous metals, requiring specialized procedures and monitoring due to the environmental hazard it presents.
Other metal methacrylates that may be useful as an ionomeric crosslinker are zirconium based compounds, such as zirconium methacrylate, Zr(MA)4, or zirconyl dimethacrylate, ZrO(MA)2. Zirconium is a fairly abundant element, is not listed on the EPA list of hazardous metals, and some zirconium precursors are commercially available at costs lower than that of zinc containing precursors. These precursors may be in powder form, which can provide difficulty in measuring and handling of the materials, and are insoluble in styrene.
In view of the above, it would be desirable to have a method for the in-situ preparation of zirconium methacrylate compounds from relatively inexpensive precursors that can be used in conjunction with styrene polymerization.